Minimally invasive interventions in the human body or even in animals are carried out as interventional methods with the aid of medical instruments such as for example catheters or guide wires. The area in which the introduced catheter and/or the introduced guide wire is/are located cannot be viewed directly by the person who is carrying out the examination. Determination of the current spatial position of the medical instrument is, however, necessary in order to convey the instrument optionally to the correct location for a treatment or to obtain information about a defined area in the body of the examination object. The medical instrument is therefore as a rule localized by means of an X-ray system, for example by fluoroscopy or angiography.
However, a problem with localization by means of an X-ray system is that as a rule no three-dimensional information can be obtained about the position of the catheter, only projection images without depth information. Furthermore, the patient is often exposed to a considerable X-ray dose.
The localization of catheters is of particular importance in interventional methods in the electrophysiological field. For example, cardiac arrhythmias are treated with the aid of electrophysiological interventions, whereby areas of defective cardiac conduction are obliterated in a targeted manner. To achieve this, medical instruments such as catheters have to be navigated in the three-dimensional cavities of the heart, while on the other hand the interventions are often prolonged and in the case of X-ray localization the long duration means that high radiation doses have to be applied.
In order to avoid disadvantages of this type, other non-X-ray-based localization methods have been proposed which have become established in particular in the aforementioned field of electrophysiology. Examples which can be cited include systems which are based on electromagnetic locating and are structured similarly to the global positioning system or the like. Besides these, there are proposals to locate medical instruments for use in electrophysiology via a simple measurement of the resistance of the body of the examination object. In each case, two electrodes on mutually perpendicular axes are stuck to the skin of the patient. A known voltage is applied between the two electrodes, the voltage then being measured at an electrode of the medical instrument. It is assumed that inside the patient the voltage drops in a linear manner so that from three voltages for three perpendicular electrode axes, which voltages are measured in succession on the medical instrument, the three spatial coordinates of the catheter can be inferred. Such a method is described in U.S. Pat. No. 5,697,377. In contrast to methods which are based on electromagnetic locating, no special and thus expensive catheters are required for this. A proportionality constant between voltage and spatial distance is determined for the locating by a calibrating method.
In the method of measuring the resistance of the body, there is, however, the problem that in reality the voltage drop does not take place in a linear manner but depends on the individual geometry and conductivity distribution in the body. Localization which is achieved through measurement of the resistance of the body as outlined above is therefore not exact with deviations in the heart, for example, of several millimeters, which can give rise to problems for examinations or treatments. In addition, a total of at least six additional electrodes have to be stuck onto the patient, which increases the time expended on conducting the examination.